This invention relates to magnetic actuators, and more particularly relates to methods for generating actuation with magnetic field-actuated materials.
Magnetic actuation is an increasingly important actuation technique for a wide range of applications. There have been identified and developed a number of magnetic materials that produce large actuation strain, as well as appreciable actuation force, with rapid actuation frequency. Such attributes address common requirements of many modern applications for which hydraulic, pneumatic, electrostatic or electro-active actuation is found to be inadequate.
The term “magnetic actuator” herein refers to a device that is capable of producing an output strain, i.e., actuation strain, in response to an appropriate magnetic driving force, by means of a magnetic field-actuated material. As such, a magnetic actuator includes a magnetic drive mechanism that produces the magnetic driving force, and an active magnetic material that transduces magnetic driving energy into output mechanical work, i.e. force and displacement. Examples of active magnetic materials include magnetostrictive materials and ferromagnetic shape memory alloy (FSMA) materials.
Application of a magnetic field to a magnetostrictive material causes the material to be strained as the magnetization vectors of the material rotate to align with the direction of the applied magnetic field. The unit cells of the material are strained by this magnetization rotation but their orientation is not changed; this results in production of an output strain. The forces developed by magnetostrictive materials can be considerably higher than those of conventional piezoelectric materials, and the energy densities supported by such magnetostrictive materials can be much larger than that of conventional hydraulic actuation systems.
Application of a stress or a magnetic field of appropriate orientation and intensity to a FSMA material in the martensitic phase causes shears in the constituent crystal or crystallites at the level of the structure's unit cells. The crystal and its sheared counterpart are twin-variants of the same structure, and have definite and differently-oriented directions of easy magnetization and spatial orientation. Twin variants can coexist in the FSMA. Their relative volume fractions are the result of the magnetic and stress-fields applied. Thus a first twin variant grows at the expense of a second when the easy axis of the first variant is better aligned with the applied magnetic field and/or its short crystallographic axis is better aligned with an applied compressive, i.e., deviatoric stress. This twin variant growth results, on a macroscopic scale, in a shape change of the FSMA material, leading to an actuation stroke. FSMA materials are generally characterized as enabling a large actuation stroke at a high speed and with a relatively high strain.
For many applications, once a magnetic actuation material is actuated by an applied magnetic field to, e.g., generate an output force or advance an actuation stroke, the material is then to be reset to a starting condition for a subsequent actuation cycle. In general, the reset process and reset conditions are specific to each magnetic actuation material. For example, magnetostrictive materials can be reset by removal of an applied magnetic field, allowing rotated domain magnetization vectors to reorient back to their original, non-actuated, orientation. This results in release of the developed actuation strain. A subsequent magnetostrictive actuation cycle can then be initiated for redevelopment of actuation strain. During the reset phase of the cycle, the magnetostrictive material can be compressed to enhance the subsequent actuation response.
FSMA materials can be reset by removal of the actuation field and application of a magnetic field in a direction that causes turning of the twin variant magnetization vectors back toward their original orientation. This reverses the magnetic field-induced extension stroke of the material caused by preferential twin variant growth. Alternately, an FSMA actuator material can be reset by mechanically forcing the material back to its original shape in the absence of the actuation field.
As evidenced by these examples, whatever magnetic material is employed in a magnetic actuator, in general some material reset mechanism is required to enable multiple reciprocating actuation cycles. Conventionally, this has required the inclusion of multiple bulky magnets or mechanical elements located at separate positions either in proximity to an actuation material or at locations to which the material is physically moved. The resulting magnetic actuation systems, tending to be large, weighty and cumbersome, cannot meet the needs of many important actuation applications.